A conventional transistor structure is known in which a layer made of Si:C or SiGe:C, etc., is formed on a region into which B (boron) as a channel impurity is diffused, and a Si layer into which an impurity is not implanted intentionally is formed thereon. This structure, for example, is disclosed in documents of Hong-Jyh Li et al., “Mat. Res. Soc. Symp. Proc.”, vol. 737, p. 643, 2003. and F. Ducroquet et al., “2004 IEDM Technical Digest.”, p. 437.
According to the transistor described in documents, since diffusion of B is suppressed in a Si:C layer, diffusion of B into the Si layer is suppressed in a channel region, and it is thereby possible to form a channel region having a steep impurity concentration distribution.